TCPDump, a powerful command-line packet analyzer; and libpcap, a portable C/C++ library for network traffic capture. It prints out a description of the contents of packets on a network interface that match the boolean expression. The Packet Capture library provides a high level interface to packet capture systems. All packets on the network, even those destined for other hosts, are accessible through this mechanism.   

TCPDump -  Network Packet Analyzer

LibreNMS is an autodiscovering PHP/MySQL/SNMP based network monitoring which includes support for a wide range of network hardware and operating systems including Cisco, Linux, FreeBSD, Juniper, Brocade, Foundry, HP and many more.
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Its features include: 
<ul>
<li>Automatically discover your entire network using CDP, FDP, LLDP, OSPF, BGP, SNMP and ARP. </li>
<li>Highly flexible alerting system, notify via email, irc, slack and more. </li>
<li>A full API to manage, graph and retrieve data from your install. </li>
<li>Generate bandwidth bills for ports on your network based on usage or transfer. </li>
<li>Stay upto date automatically with bug fixes, new features and more.</li>
<li>Horizontal scaling to grow with your network.</li>
</ul>

LibreNMS is an autodiscovering PHP/MySQL/SNMP based network monitoring which includes support for a wide range of network hardware and operating systems including Cisco, Linux, FreeBSD, Juniper, Brocade, Foundry, HP and many more.

LibreNMS - Network monitoring system

Modern, powerful open source C++ class libraries and frameworks for building network- and internet-based applications that run on desktop, server, mobile and embedded systems. It has a collection of C++ class libraries, conceptually similar to the Java Class Library, the .NET Framework or Apple’s Cocoa. 

POCO C++ Libraries - Cross-platform C++ libraries with a network/internet focus

Snort is an open source network intrusion prevention and detection system (IDS/IPS) developed by Sourcefire. Snort can perform protocol analysis and content searching/matching. It can be used to detect a variety of attacks and probes, such as buffer overflows, stealth port scans, CGI attacks, SMB probes, OS fingerprinting attempts, and much more.
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Snort has a real-time alerting capability as well, incorporating alerting mechanisms for syslog, a user specified file, a UNIX socket, or WinPopup messages to Windows clients. Snort has three primary uses: a straight packet sniffer like tcpdump, a packet logger (useful for network traffic debugging, etc), or a full-blown network intrusion prevention system.

Snort is an open source network intrusion prevention and detection system (IDS/IPS) developed by Sourcefire. Snort can perform protocol analysis and content searching/matching. It can be used to detect a variety of attacks and probes, such as buffer overflows, stealth port scans, CGI attacks, SMB probes, OS fingerprinting attempts, and much more.

Snort - Network Intrusion Prevention and Detection System

Robust, Lightweight, Cross-platform, Template Software. STLSoft is a collection of STL and standard library extensions, and facades over operating-system and 3rd-party APIs.

STLSoft - C and C++ Libraries

Flannel runs a small, single binary agent called flanneld on each host, and is responsible for allocating a subnet lease to each host out of a larger, preconfigured address space. Flannel uses either the Kubernetes API or etcd directly to store the network configuration, the allocated subnets, and any auxiliary data (such as the host's public IP). Packets are forwarded using one of several backend mechanisms including VXLAN and various cloud integrations.

flannel - Network fabric for containers, designed for Kubernetes

Open vSwitch is well suited to function as a virtual switch in VM environments. In addition to exposing standard control and visibility interfaces to the virtual networking layer, it was designed to support distribution across multiple physical servers. Open vSwitch supports multiple Linux-based virtualization technologies including Xen/XenServer, KVM, and VirtualBox.

Open vSwitch - Multilayer Virtual Switch

IPFire is a server distribution with intended to use as a firewall. It focuses on flexibility, and scales from small to middle sized business networks and home networks. Beginning with a small firewall system of a few megabytes, it is possible to run IPFire as a file server or VPN gateway for staff, branches or customers. This modularity means that yor version of IPFire runs with exactly what you require and nothing more.

IPFire - An open source firewall solution

pfSense is a powerful, flexible firewalling and routing platform. It includes a long list of related features and a package system allowing further expandability without adding bloat and potential security vulnerabilities to the base distribution.
  pfSense is a stateful firewall, by default all rules are stateful. A Dynamic DNS client is included to allow you to register your public IP with a number of dynamic DNS service providers.

pfSense - Firewall and Routing platform

SmoothWall is a open source Firewall distribution. It Supports LAN, DMZ, and Wireless networks, Static Ethernet, DHCP Ethernet, PPPoE, PPPoA using various USB and PCI DSL modems. It provides traffic stats, POP3 email proxy with Anti-Virus, Web proxy for accelerated browsing and lot more.

SmoothWall - Express Open Source Firewall Project

Apache MINA is a network application framework which helps users develop high performance and high scalability network applications easily. It provides an abstract, event-driven, asynchronous API over various transports such as TCP/IP and UDP/IP via Java NIO.

Apache MINA - A network application framework

IPCop provides secure and stable Linux Firewall Distribution. It is a complete Linux Distribution whose sole purpose is to protect the networks it is installed on. It is geared towards home and SOHO users. The IPCop web-interface is very user-friendly and makes usage easy.

IPCop - Linux firewall distribution

ScanSSH supports scanning a list of addresses and networks for open proxies, SSH protocol servers, Web and SMTP servers. Where possible ScanSSH, displays the version number of the running services. ScanSSH protocol scanner supports random selection of IP addresses from large network ranges and is useful for gathering statistics on the deployment of SSH protocol servers in a company or the Internet as whole. 

ScanSSH -  Scans the network for open proxies

Meerkat-Monitor is a simple yet powerful monitoring tool for applications and services. It’s good for local or cloud environment deployment. Supports monitoring of several services like Databases, Websites, Network Sockets, Webservices and SSH. It’s very dynamic and allows you to take advantage of monitoring through several types of tasks (like executing DB queries or remote commands).

Meerkat Monitor - Network Monitoring Tool

Secure Socket Funneling (SSF) is a network tool and toolkit. It provides simple and efficient ways to forward data from multiple sockets (TCP or UDP) through a single secure TLS link to a remote computer. SSF also allows dynamic port forwarding with a SOCKS server from the client to the server  and from the server to the client.

Secure Socket Funneling - Network tool and toolkit

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TCPDump - Network Packet Analyzer